Myelin sheath thickness in the CNS is regulated near the axon

Two myelin sheaths sharing a common outer tongue process are illustrated in a thin section electron micrograph from mouse optic nerve. Although they derive from the same outer tongue process, and therefore from the same oligodendrocyte, the two sheaths are of different thicknesses (6 and 9 turns). The example demonstrates that myelin sheath thickness is regulated independently for each axon, at a site very near the axon.